亚洲激情+欧美激情,无码任你躁久久久久久,我的极品美女老婆,性欧美牲交在线视频,亚洲av高清在线一区二区三区

^{<blockquote id="i1nrx"></blockquote>}

電子課本網(wǎng) › 第123頁

第123頁

信息發(fā)布者：

$證明：∵四邊形???ABCD???為矩形$

$??? ∴∠B=∠C=90°，??????AB=CD???$

$??? ∵EF⊥ED ???$

$??? ∴∠BEF+∠CED=∠FED=90°???$

$??? ∵∠BEF+∠BFE=90°???$

$??? ∴∠BFE=∠CED???$

$ 在???△BFE???和???△CED???中$

$??? \begin{cases}∠B=∠C\\∠BFE=∠CED\\EF=ED\end{cases}???$

$??? ∴△BFE≌△CED(\mathrm {AAS})???$

$??? ∴BE=CD???$

$??? ∴AB=BE，???$

$??? ∴∠BAE=45°???$

$??? ∴AE???平分???∠BAD???$

$證明：???(1)???由翻折的性質(zhì)可得，???∠ABE=∠EBM=\frac 12∠ABM，??????∠BDF=∠CDF=\frac 12∠BDC???$

$??? ∵AB//CD???$

$??? ∴∠ABD=∠BDC???$

$??? ∴∠EBM=∠BDF???$

$??? ∴BE//DF???$

$??? ∵AD//BC???$

$ ∴四邊形???BFDE???為平行四邊形$

$??? (2)∵???四邊形???BFDE???為菱形$

$??? ∴BF=DF???$

$??? ∴∠FBD=∠BDF???$

$??? ∵∠FBD+2∠BDF=90°???$

$??? ∴∠FBD=30°???$

$??? ∵AB=CD=2???$

$??? ∴BD=4?$

$?∴AD=\sqrt{42-22}=2\sqrt{3}???$

$?∴BC=AD=2\sqrt{3}.?$